1. Field of the Invention
This document relates to an organic light emitting diode (OLED) display device that can compensate for a threshold voltage of a driving thin film transistor (TFT), a voltage drop of a supply voltage, and the mobility of the driving TFT.
2. Discussion of the Related Art
In recent years there has been an increasing demand for display devices. Various flat panel displays such as liquid crystal displays (LCDs), plasma display panels (PDPs), and organic light emitting diodes (OLED) displays have been widely used to meet this demand. Compared to other flat panel displays, OLED display devices are driven at a lower voltage, are thinner, have a wider viewing angle and a quicker response speed.
One specific type of OLED display device is an active matrix OLED display device. Active matrix OLED display devices have a plurality of pixels disposed in a matrix form to display an image. The plurality of pixels of an active matrix OLED display are defined by scan lines and data lines. Each pixel includes a scan thin film transistor (TFT) supplying data voltages from data line in response to scan signal from the scan line. Each pixel also includes a driving TFT controlling the amount of current supplied to an OLED in response to the data voltage supplied to a gate electrode of the driving TFT. The current Ids between the drain and source of the driving TFT supplied to the organic light emitting diode can be represented by Equation 1:Ids=k′·(Vgs−Vth)2  [Equation 1]
In equation 1, k′ indicates a proportionality factor determined by the structure and physical properties of the driving TFT, Vgs indicates a voltage difference between a gate and a source of the driving TFT, and Vth indicates a threshold voltage of the driving TFT.
However, due to a threshold voltage shift caused by deterioration of the driving TFT, the threshold voltage Vth of the driving TFT of each of the pixels has a different value. The current Ids between the drain and source of the driving TFT is dependent on the threshold voltage Vth of the driving TFT. Thus, the current Ids between the drain and source of the driving TFT of each pixel varies even if the same data voltage is supplied to each of the pixels. Accordingly, there arises the problem that the luminance of light emitted from the OLED of each of the pixels varies even if the same data voltage is supplied to each of the pixels. To solve this problem, various types of pixel structures for compensating the threshold voltage of the driving TFT of each of the pixels have been proposed.